This invention relates to a process for the extraction of metals from an aqueous solution of metals in the form of their metal ions. In particular, the invention relates to the preparation and use of an efficient organic extractant and also the recovery from the organic extractant of the metal ions.
Many industrial processes lead to the production of water containing one or more metal ions. In particular, the mining industry must deal with metal ion recovery from aqueous solutions. The treatment and disposal of acid mine drainage can be difficult. Acid mine drainage is an aqueous solution containing a variety of metal ions. The solution is often harmful to the environment and also to human health. Acid nine drainage is usually treated to clean the water but standard treatment processes usually lead to the production of mixtures of unwanted solid metal salts which are typically dumped. However, in time, the metal salts contaminate ground water through contact with rainwater and natural seepage.
In a typical solvent extraction process, an aqueous solution of metal ions is contacted with an organic extractant in an organic solvent. The organic solvent is typically a hydrocarbon solvent. The organic extractant typically has an available proton which can exchange with a metal ion from the aqueous solution. Such extractants include organic acids, for example naphthenic acid, and chelation extractants, for example certain hydroxy oximes. The organic acids rely on simple ionic interactions to attract metal ions from the aqueous solution in exchange for protons. The chelation extractants use a chelate effect to bind metal ions in a claw-like manner with concomitant release of protons to the aqueous solution.
The exchange of metal ions to the organic solution and hydrogen ions to the aqueous solution causes the pH of the aqueous solution to lower. In order to maintain the exchange equilibrium and prevent the exchange reaction from coming to a halt, the pH of the aqueous solution must be continually adjusted. This is particularly the case for organic acid extractants. The adjustment is usually made by the addition of ammonia or sodium hydroxide. The pH adjustment can therefore make the extraction process expensive and complex.
Following extraction of the metal ions into the organic solution, the organic solution containing extractant loaded with metal ions is then typically subjected to a stripping process. The standard stripping method involves contacting the organic solution with aqueous acid, such as sulphuric acid or hydrochloric acid, to transfer the metal ions to an aqueous solution leaving reprotonated organic extractant to be recycled for further extraction.
A typical aqueous solution to be treated will contain two or more types of metal ions, depending on the source of the aqueous solution. For example, a solution may contain copper ions and zinc ions in addition to lower concentrations of other metal ions such as nickel, manganese, and iron.
It is desirable to be able to extract the metal ions selectively from the aqueous solution. The selectivity can be controlled to an extent by maintaining the pH in a predetermined range for a particular organic extractant. For example, zinc ions can be extracted from an aqueous solution using di-2-ethylhexylphosphoric acid (D2EHPA) at a pH of approximately 3 without extracting any significant amount of manganese ions which are present in the aqueous solution. The avoidance of any coextraction means that stripping of the organic solution results in an aqueous solution containing only one type of metal ion. The metal can then be recovered in pure form using standing methods such as electrolysis.
U.S. Pat. No. 6,378,262 describes the selective recovery of metals such as nickel and cobalt from solutions additionally containing other metals such as manganese, calcium and magnesium. The process includes maintaining the pH of the aqueous solution between approximately 2 and 6 using a sodium hydroxide solution. The aqueous solution is contacted with a water-immiscible organic solution containing an extractant. Following separation of the aqueous and organic phases, the metal-loaded organic solution is contacted with an aqueous acid strip solution to recover the metal.
U.S. Pat. No. 4,423,012 describes the recovery of manganese and zinc ions from an aqueous solution. The extractant used is di-ethylhexylphosphoric acid (DEHPA). The organic solution of DEHPA is contacted with the aqueous solution of metal ions. Following separation of the aqueous and organic solutions, the organic solution is stripped of metal ions using aqueous acid. The DEHPA can be regenerated using calcium oxide or magnesium oxide. However, mixing times typically between 5 and 10 minutes are required in order to achieve sufficient loading of calcium or magnesium into the organic solution of DEHPA.
U.S. Pat. No. 5,779,997 describes the use of a phosphinic acid extractant. The extractant is firstly converted into its sodium, potassium or ammonium salt using sodium hydroxide, potassium hydroxide or ammonia, respectively. The object is to avoid the addition of sodium, potassium, or ammonium ions into feed solutions causing jarosite to precipitate. The salt of the organic acid is then contacted with a solution of magnesium ions and the magnesium salt of the organic acid is then used to extract cobalt from the feed solution. However, the use of sodium, potassium or ammonium ions is expensive and laborious.
It is therefore an object of this invention to provide a process for recovering metal ions from an aqueous solution using a calcium or magnesium loaded organic extractant, or to at least provide a useful alternative:
It is also an object of this invention to provide a process for preparing a calcium or magnesium loaded organic extractant, or to at least provide a useful alternative.
It is a further object of this invention to provide a solvent extraction process for producing a purified magnesium chloride solution essentially free of calcium ions, from magnesium feedstocks that contain calcium impurities, or to at least provide a useful alternative.
In a first aspect of the invention there is provided a process for preparing an extractant solution where the extractant solution is a solution of a calcium ion or magnesium ion loaded extractant in a water-immiscible organic solvent, and where the extractant solution is suitable for extracting metal ions from an aqueous solution containing the metal ions, including the steps:
a) mixing an aqueous solution of calcium ions or magnesium ions with a basic calcium salt or a basic magnesium salt and with a solution of an organic extractant in a water-immiscible organic solvent to form the extractant solution; and
b) separating the extractant solution from the aqueous solution.
Preferably the basic calcium salt is calcium oxide (CaO), calcium hydroxide (Ca(OH)2), or calcium carbonate (CaCO2). Preferably the basic magnesium salt is magnesium oxide (MgO), magnesium hydroxide (Mg(OH)2) magnesium carbonate (MgCO3), hydrated magnesium carbonate (MgCO3.xH2O), basic magnesium carbonate (xMgCO3.yMg(OH)2.zH2O), or dolomite (CaCO3.MgCO3).
In a preferred embodiment of the invention the extractant solution is a solution of a magnesium ion loaded extractant in a water-immiscible organic solvent. The magnesium may be recovered from this solution by treatment with aqueous hydrochloric acid to give a solution of magnesium chloride which is free of any non-magnesium metal ions.
Preferably the aqueous solution of calcium ions or magnesium ions of step a) is a solution of magnesium chloride. It is also preferred that the solution contains sulphate ions, preferably obtained from magnesium sulphate and/or sulphuric acid.
In a further preferred embodiment of the invention the process also includes the steps:
c) contacting the extractant solution with an aqueous solution containing metal ions to give a solution of the extractant loaded with some or all the metal ions in the organic solvent and to give an aqueous solution containing some or all the calcium ions or magnesium ions; and
d) separating the aqueous solution from the solution of the extractant loaded with some or all the metal ions In the organic solvent.
A buffer, such as acetic acid, a mono or diamine, an alkanolamine, or an amino acid, may be used in this invention.
Preferably the organic extractant of this invention is a carboxylic acid, an hydroxy oxime, or an organophosphorous acid.
Examples of carboxylic acids include naphthenic acid, versatic acid, (Z)-9-octadecenoic acid, isostearic acid, 2-octyl-dodecanoic acid, 2-hexyl-decanoic acid, and 2-butyl-octanoic acid. Examples of hydroxy oximes include 2-hydroxy-5-nonylacetophenone oxime, 5-dodecylsalicylaldoxime, and 5-nonylsalicylaldoxime. Examples of organo-phosphorous acids include di-(2-ethylhexyl)phosphoric acid (DEHPA), 2-ethylhexyl phosphonic acid mono-2-ethylhexyl ester (IONQUEST 801), and bis(2,4,4-trimethylpentyl)phosphinic acid (CYANEX 272).
The aqueous solution of calcium ions of step a) of the first aspect of the invention may be formed from calcium chloride (CaCl2) or calcium nitrate (Ca(NO3)2), whereas the aqueous solution of magnesium ions may be formed from magnesium nitrate (Mg(NO2)2), magnesium sulphate (MgSO4), or magnesium chloride (MgCl2).
Typically the metal ions which may be extracted using the process of this invention are iron (Fe+2 or Fe+3), aluminium (Al+3), cobalt (Co+2), copper (Cu+2), mercury (Hg+2), nickel Ni+2), zinc (Zn+2), manganese (Mn+2), lead (Pb+2) or cadmium (Cd+2) ions, or any mixtures or combinations thereof.
The water-immiscible organic solvent is preferred to be an aliphatic or aromatic hydrocarbon solvent, or mixtures thereof. Preferably the water-immiscible organic solvent is an industrially available high flash point aliphatic and/or aromatic solvent. Examples include Exxsol D80 (aliphatic), Recosol 150 (aromatic) and Shellsol 2046 (aliphatic/aromatic blend).
An another aspect of the invention there is provided an extractant solution loaded with calcium ions or magnesium ions.